1. Field of the Invention
The present invention relates to the field of earth boring tools and more particularly to rotating drag bits and the cutters contained thereon.
2. Description of the Related Art
Drilling in shale or plastic formations with a drag bit has always been difficult. The shale, under pressure and in contact with hydraulics, tends to act like a sticky mass, sometimes referred to as gumbo, which balls and clogs the bit. Once the bit balls up, it ceases to cut effectively.
One type of drag bit includes polycrystalline diamond compact (PDC) cutters which present a generally planar cutting face having a generally circular perimeter. A cutting edge is formed on one side of the cutting face which, during boring, is at least partially embedded into the formation so that the formation is received against at least a portion of the cutting surface. As the bit rotates, the cutting face moves against the formation and a chip, which rides up the surface of the face, forms. When the bit is functioning properly, the chip breaks off from the remainder of the formation and is transported out of the bore hole via circulating drilling fluid. Another chip begins to form, also sliding up the face of the cutting surface and breaking off in a similar fashion. Such action occurring at each cutting element on the bit causes the bore to become progressively deeper.
In low permeability formations, however, drilling fluid is not transported far into the formation. There can thus be a pressure difference in the range of 20,000 psi between the well bore, which is under pressure from the drilling fluid, and the rock pores near the bore. As the bit rotates, rock pore pressure appears between that portion of the cutting face embedded into the formation and the chip riding up the cutting face. Because well bore pressure appears on the other side of the chip it is effectively plastered against the cutting surface by the pressure differential. Friction between the chip and the face of the cutter increases proportional to the pressure differential across the chip. Thus, when there is a high pressure differential, the chip is compressed by a force generated by the pressure differential across the chip which acts to increase friction for opposing the direction of the sliding chip on the face of the cutter. The sliding movement of the chip over the cutter is thus slowed and the bit becomes balled and clogged by the rock being bored. Furthermore, bit balling compresses the formation being cut thus making cutting more difficult.
Although not all prior art cutting element surfaces are planar, none are known which provide fluid communication to a location closely adjacent that portion of the cutting surface embedded in the formation thereby relieving the pressure differential across the chip. For example, U.S. Pat. No. 4,872,520 to Nelson discloses a flat bottom drilling bit with polycrystalline cutters. These cutters are shaped to provide a cutting edge which does not wear flat even when the cutter is worn. U.S. Pat. Nos. 4,558,753; 4,593,777; and 4,660,659 similarly disclose a drag bit and cutters which maintain a sharp cutting edge even as the cutting elements wear. U.S. Pat. No. 4,984,642 to Renard et al. utilizes a cutter having corrugations formed thereon. These corrugations, however, are defined by gradually sloping walls having an angle of approximately 45 degrees relative the cutting surface. This structure permits rock to be urged into the corrugations and against the walls thereby enabling a high pressure differential across rock chips cut by the bit and thus causing the resulting problems as described above.